Daniel Grin Awarded NASA Grant

Assistant Professor of Physics and Astronomy Daniel Grin, who was named a Kavli Institute for Theoretical Physics (KITP) Scholar last summer, now has another accolade to add to his résumé. He, along with Swarthmore’s Tristan Smith, has been named a recipient of one of NASA’s 2018 Astrophysics Theory Program (ATP) grants, which “support efforts to develop the basic theory for NASA’s space astrophysics programs.”

This year, 219 applications were submitted for consideration, of which only 51 were selected. Worth a hefty $436,808, of which $232,008 will go to Haverford College, Smith and Grin’s award will be used to marry astrophysical observations and fundamental particle physics theory to further their research on the physics of dark matter and dark energy.

“It will fund six undergraduate summer student stipends at each institution over the life of the grant, three years of summer salary for both myself and Professor Smith, and half of my pre-tenure leave-year salary,” Grin says. “The grant will also fund travel to visit collaborators and attend conferences for both the principal investigators and students involved in the research.”

Grin explains dark matter and dark energy as exotic forms of matter and energy. “Dark matter can be thought of as gravitational glue holding together galaxies,” he says, “[and] dark energy… is an even more mysterious substance, which dominates the universe today and is thought to be responsible for the fact that the universe’s expansion has begun to accelerate.”

Though scientists have not yet managed to directly detect the presence of dark matter and dark energy (though efforts utilizing everything from particle colliders to gamma-ray telescopes to underground experiments are underway), Grin says, their existence is necessitated by current cosmological observations, because baryonic matter and radiation—that is, the usual forms of matter and energy “we, and everything we interact with on a day-to-day basis, are made of”—only account for five or so percent of the universe’s makeup.

Grin and Smith are coming at the problem of detection by a novel route: They are treating the entire “dark” sector of the universe—which includes very light particles known as neutrinos in addition to dark matter and dark energy—“as a single dark fluid, with relationships between physically intuitive quantities like pressure, shear, and density set by free mathematical functions,” as well as a technique known as ‘principal component analysis.’ Rather than utilize existing models of dark fluids, they plan to use the data they extract from those mathematical functions to build entirely new models of the dark sector.

Grin likens the process of testing dark sector models using cosmological observations alone to “throwing darts at a dartboard and seeing which ones stick.”

“So [this involves] trying one idea at a time,” he says. “‘Oh, does that work—yes, no?’ [And if ‘No,’ then back to the drawing board.] But the point is that it’s painstakingly exhaustive because you can test one idea at a time—that’s the best you can do. So what Tristan and I are doing instead is looking at the recession velocities of distant objects, the inhomogeneities of the microwave afterglow of the Big Bang, and the spatial distribution of galaxies today to reverse-engineer the process and see what physical properties of dark matter, dark energy, and neutrinos can be gleaned from the data.”

Though Grin’s not sure if this methodology will work, his attitude towards the possibility of failure is surprisingly sanguine.

“It may not pan out, but it’s worth a try,” he says. “Tristan and I are trying to turn things around a little—take our wisdom from the experimental physicists and the observational astronomers and figure out what the world tells us about dark matter instead of trying to shove our ideas into the data.”

Though more Haverford students will eventually join this research initiative, Grin will be aided only by rising senior Maxwell Aifer ’19 this summer. Aifer, a physics major himself, “will be working on testing how well [a formalism Smith and Grin plan to make use of known as] 'generalized dark matter' can mimic the current preferred cosmological model, the so-called Lambda CDM model”—research that may be the foundation of his senior thesis.

Grin and his collaborators’ new methodology could provide “libraries of tools for theorists to test models against data-driven functions." "This,” says Grin, “could help construct a new data-driven, model-independent approach of great use to the dark matter and dark energy research community.” In following in the investigative footsteps of fellow Haverford professors Bruce Partridge and Steve Boughn, he is continuing a longstanding tradition of conducting research on the greater cosmos.